1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system which is adapted to control the air-fuel ratio of a mixture supplied to the engine to a desired air-fuel ratio, based on outputs from exhaust gas ingredient concentration sensors arranged in an exhaust passage of the engine.
2. Prior Art
It is conventionally known to arrange an exhaust gas ingredient concentration sensor (hereinafter referred to as "the LAF sensor") having an output characteristic which is substantially proportional to the concentration of an exhaust gas ingredient, in an exhaust passage of an engine, and to feedback-control the output from the LAF sensor to a value corresponding to a desired air-fuel ratio of an air-fuel mixture supplied to the engine.
However, according to this technique of the air-fuel ratio feedback control, when the desired air-fuel ratio is set to a stoichiometric air-fuel ratio (A/F=14.7), it is often actually difficult to converge the air-fuel ratio of a mixture to the stoichiometric air-fuel ratio due to an error or tolerance in the output from the sensor caused by an amplifier circuit connected to the LAF sensor, which results in degraded emission characteristics. Therefore, it is required to set a desired air-fuel ratio coefficient corresponding to the stoichiometric air-fuel ratio to a value slightly deviated from 1.0, engine by engine, on shipment thereof.
To eliminate such an inconvenience, an air-fuel ratio control system has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 2-67443, which comprises a LAF sensor arranged in an exhaust passage of an engine at a location upstream of a catalytic converter, and an O2 sensor arranged in same at a location downstream of the catalytic converter, an output from which drastically changes when the air-fuel ratio of a mixture supplied to the engine changes across the stoichiometric air-fuel ratio, wherein the desired output voltage of the LAF sensor or desired air-fuel ratio coefficient is corrected based on an output from the O2 sensor in controlling the air-fuel ratio to the stoichiometric air-fuel ratio, whereby the output from the LAF sensor is feedback-controlled to the corrected desired output voltage or an equivalent ratio of the output from the LAF sensor is feedback-controlled to the corrected desired air-fuel ratio coefficient.
According to the proposed air-fuel ratio control system, it is possible to perform an accurate air-fuel ratio control to the stoichiometric air-fuel ratio based on the output from the O2 sensor by always causing the desired output voltage from the LAF sensor or the desired air-fuel ratio coefficient to assume a value actually corresponding to the stoichiometric air-fuel ratio.
However, in this conventional air-fuel ratio control system, if the output from the O2 sensor falls within a predetermined particular range during the air-fuel ratio feedback control to the stoichiometric air-fuel ratio, it means that the air-fuel ratio of a mixture supplied to the engine has been controlled to the stoichiometric air-fuel ratio and hence that the desired output voltage from the LAF sensor and the desired air-fuel ratio coefficient assume respective values substantially accurately corresponding to the stoichiometric air-fuel ratio by this conventional system. Nevertheless, during the air-fuel ratio control to the stoichiometric air-fuel ratio, the air-fuel ratio of the mixture is always feedback-controlled based on the output from the O2 sensor (this specific air-fuel ratio feedback control to the stoichiometric air-fuel ratio based on the output from the O2 sensor will be hereinafter referred to as "the O2 feedback control"). In other words, although the air-fuel ratio of the mixture can be controlled to the desired air-fuel ratio, i.e. to the stoichiometric air-fuel ratio without the O2 feedback control, the O2 feedback control is unnecessarily carried out, which can result in all the more degraded air-fuel ratio controllability in the aforementioned predetermined range, e.g. due to fluctuation in the desired output voltage from the LAF sensor or the desired air-fuel ratio coefficient, preventing the air-fuel ratio feedback control from being executed in a desired manner.
Further, even if the O2 feedback control is carried out when there is a large difference between an actual value of the output from the O2 sensor and a value of same corresponding to the stoichiometric air-fuel ratio, e.g. when the output from the O2 sensor is lower than a predetermined lower limit value, or higher than a predetermined higher limit value, it is difficult to quickly converge the air-fuel ratio of the mixture to the stoichiometric air-fuel ratio, and in the worst case, there is a possibility of diverging the air-fuel ratio of the mixture. In other words, even if the feedback control is carried out when the output from the O2 sensor is lower than the predetermined lower limit value, the control system can only exhibit a poor air-fuel ratio converging characteristic, causing an undesired emission of NOx, while even if the feedback control is carried out when the output from the O2 sensor is higher than the predetermined higher limit value, this gives rise to an undesired emission of CO and HC for the same reason, in both cases, resulting in degraded exhaust emission characteristics of the engine.